This invention relates to a process for making antimony pentoxide.
Colloidal antimony pentoxide is frequently used as a metals passivation additive and a specialty fire retardant.
U.S. Pat. No. 4,348,301 discloses a means of making hydrous antimony pentoxide by contacting hydrogen peroxide and an antimony trioxide aqueous slurry in a batch system with and without a stabilizer. The stabilizer is generally an alkanolamine, alkanolamine salt, alpha-hydroxycarboxylic acid or a polyhydroxy alcohol and reportedly functions as a catalyst thereby increasing the reaction rate and producing a colloidal suspension of smaller average particle size. As previously noted, this prior art discloses use of the reaction in a batch reactor system. In such a system, the reactants are initially loaded into a vessel where they are well mixed and remain until the desired degree of conversion is obtained. The resultant mixture is then discharged. While the reaction is ongoing, the composition or degree of conversion is changing with time but at any point within the reactor, the composition is generally uniform. Batch reactors are extremely simple to operate and are frequently used for the preparation of small quantities of specialty chemicals. However batch reactors are limited in throughput capacity, are difficult to scale-up, and are often energy and manpower intensive.
Because of the inherent limitations associated with the operation of a batch reactor, continuous flow processes are frequently preferred when possible. One alternative used by those skilled in the art is to place tank reactors in series wherein the effluent stream from the upstream reactor becomes the influent stream to the downstream reactor. Each tank possesses a reactant of progressively greater conversion and at steady-state conditions, the degree of conversion in each tank becomes a fixed value. However, the residence time of the reactant species in a given tank may differ significantly as reactant which has just entered the tank is mixed with reactant which has been there for a significant period of time. This phenomenom is referred to as back-mixing. The high degree of mixing in each tank assures a uniform overall composition and the effluent from a given tank is representative of the actual composition within the tank. Limitations associated with the tank reactors in series include the need for many tanks when high conversion is desired, the process equipment is expensive to buy and to maintain, the high degree of mixing requires significant energy input, the significant difficulties exist with repsect to process scale-up.
A second approach for a continuous flow reactor system is to inject the reactants into a pipe (i.e., a tubular reactor) of sufficient length and obtain the desired product in the produced effluent. Tubular reactors are easy to design and operate and inexpensive to construct. However, non-uniform velocity distributions, radial temperature gradients and poor radial mixing can limit practical applications when high viscosity fluids are involved. U.S. Pat. No. 4,022,710 discloses hydrous antimony pentoxide production via the reaction of hydrous antimony trioxide with hydrogen peroxide without a stabilizer but in a continuous flow, fixed diameter reactor. The desired antimony trioxide concentration in the feed is stated to be 1 to 20 wt %, with 5 to 10 wt % being preferred. A hydrogen peroxide to antimony trioxide mole ratio of not less than 3 and preferably 5 to 10 is taught. A nominal operating temperature of 90.degree. C. is disclosed. To obtain a colloidal product of desired particle size and to avoid plugging of the reactor, this art discloses the requirement that fluid mixing in the reactor be minimized and that the internals of the reactor be constructed of a non-wetting material. To minimize fluid mixing, the art requires all bends be removed from the system and the operation at flow velocities which minimize fluid mixing. Problems associated with the plugging of the reactor were apparently resolved by constructing the reactor of a non-wetting resin, preferably tetrafluoroethylene, rather than stainless steel. From a practical perspective, these restrictions significantly increase the reactor cost on a per unit throughout basis.
Although the art is silent, the Examples and operational restrictions cited in `710` indicate that process operation was restricted to the laminar flow regime (Reynolds Number &lt;2000) and that these conditions were incorrectly referred to as "plug flow" (a possible Japanese to English translation error). For smooth circular pipes and Newtonian fluids, those skilled in the art recognize a departure from laminar flow conditions when a dimensionless number, DV.rho./.mu., is greater than approximately 2000. This dimensionless group is referred to as the Reynolds number wherein D is the pipe diameter, V is the superficial velocity defined as the total volumetric flow rate (Q) divided by the cross-sectional area available to flow (A), .rho. is the bulk fluid density, and .mu. is the bulk fluid viscosity. The art teaches that a transition zone from laminar to turbulent flow exists for Reynolds Numbers between 2000 and 4000 and that turbulent flow exists at Reynolds Numbers greater than 4000.
When operating in the laminar region, fluid flow is solely in the axial direction and fluid mixing is minimal and primarily by diffusional effects. The lack of mixing restricts heat transfer and can result in nonuniformities in temperature which can result in nonuniformities in reaction rate and product produced. The velocity profile is a maximum at the center of the pipe and decreases in a parabolic manner to zero at the wall. Therefore when a slug of fluid is injected into the pipe, the fluid injected at the center will be produced well before that injected near the wall. When operating at laminar flow conditions, the residence time of a given fluid element when flowing through the pipe will be dependent on the point of injection on the entrance cross-section.
When operating in the turbulent flow regime, chaotic mixing is superimposed on the bulk axial flow. As a result, the velocity profile from the center of the pipe to the wall is nearly constant. This results in nearly uniform compositions and temperatures at a given radial cross-section and all fluid elements will have similar residence times regardless of where injected on the entrance cross section. As assemblage or slug of fluid elements simultaneously injected into the tube at turbulent flow conditions will advance like a plug through the pipe. In the literature, this condition is routinely referred to as "plug flow".